Frequency shift signaling system



Dec. 27, 1949. H. E. GOLDSTINE I FREQUENCY SHIFT SIGNALING SYSTEM 2Sheets-Sheet 1 Filed Jan. 29, 1946 INVENTORD flallanlfialdztuw,

Q ATTORNEY Dec. 27, 1949 H. E. G'OLDSTINE 2,492,795

FREQUENCY SHIFT SIGNALING SYSTEM Filed Jan. 29, 1946 2 Sheets-Sheet 2igga 055 701v: Kim) SEA Him 7011/5 JKEYi/Z mr/flaz I I;

INVENTOR BY n g/ww ATTORNEY mitted. Obviously,

Patented Dec. 27, 1949 FREQUENCY smrr SIGNALING SYSTEM Hallan E.Goldstine, Port Jefferson, N. Y., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application January 29; 1946, SerialNo. 644,147

11 Claims.

This application discloses an improved method of and means forproducingtelegraphy signals or facsimile signals of the frequency shift type, orof the on-off type or telephony signals.

Frequency shift signalling systems are known in the prior art and thenature of these signals and known method of producing the same will notbe discussed in detail herein. Frequency shift signals comprise twocurrents of different frequency alternatively usable. The shifting fromone current to the other current is usually in accordance withintelligence represented by code. The current frequencies differ byseveral hundred cycles, and one may represent mark and the other spacewhen telegraphy signals are being sent. When facsimile signals areinvolved one frequency may represent black or white. The character oftelegraphy or facsimile systems of the on-oif or amplitude modulationtype are well known, as is the character of amplitude modulationtelephony signals, and the same will not be described herein.

In frequency shift systems, as stated above, current is shifted from onefrequency to the other in accordance or the same impressed on carriercurrent is transin view of the already crowded condition of theradio-frequency spectrum it is of considerable importance to have asystem of 'this type wherein the mean or averbe maintained conage orcenter frequency can stant, so that the frequency shifted current willnot deviate from its assigned band in the frequency spectrum. An objectof my invention is to provide center frequency is fixed. In my improvedsysterm this is accomplished by modulating an unstable oscillator from afirst frequency to a second frequency and using the oscillators outputto modulate a crystal controlled oscillator of fixed frequency. The sumor difference side band is then selected for use at the fundamentalfrequency or a harmonic thereof.

My frequency shift means is to comprise a unit for use in new orexisting transmitter sys-- terns. These transmitter systems may have ormay be provided with crystal oscillators supplying the carrier currentof fixed frequency used in providing frequency shifted side band energyof fixed center frequency. The modulated oscillator also must have theproper stable center frequency about which it is shifted to provide 1efinal desired output frequency which may be harmonically related to theselected side band frequency which in the embodiment described is withsignals and this current an improved system wherein the tone keyed inaccordance the sum of the crystal frequency and the modulated oscillatorfrequency. A further object of my invention is to provide means fortuning the modulated oscillator to the exact desired center frequencywithout putting the transmitter on the air. This permits adjustment ofthe frequency shift unit in the laboratory and is accomplished bsupplying a crystal controlled monitor oscillator of a frequency equalto the desired center frequency of the modulated oscillator andarranging the circuits so that the modulated oscillator frequency may becompared with and brought into synchronism with the crystal controlledoscillator frequency. This crystal controlled monitoring oscillator isalso made use of as the modulated oscillator when amplitude keying oramplitude modulated telephony signals are to be generated. 7

Many telegraphy systems today make use of a with intelligence. The toneis rectified to provide voltage having a direct current componentvarying between zero potential and a negative value, say minus 180volts. The so varying direct current potential is then used to key thetelegraphy system. My frequency shift unit is so arranged that it may becontrolled by the output of these existing keyed tone converters.However, the output of the keyers supplying the tone for conversion issubject to variations due to variations in power supply voltage. Thisresults in variations in the potential at the converter output. Sincethis tone converter output varies between about zero and a negativevalue signal limiting is inherent in the positive direction. The directcurrent varying potential in said prior systems is for use in amplitudekeying and in such use signal limiting in the negative direction is notso important and is provided in the system. In my system, however, thisdirect current potential is to control a reactance tube modulating anoscillator and it is important that the signalling be also limited inthe negative direction. A further object of my invention is to providemeans which in cooperation with the existing keyed tone convertersaccomplishes signal limiting in both directions of the frequency shiftoperation. In accordance with my invention these variations are removedby provision of a gas discharge voltage regulator tube shunting thekeyed tone converter output which is also in effect the reactance tubecontrol input.

In on-oif telegraphy systems the transmitted wave by necessity covers aconsiderable frequency band'since the keying on and off of the carriercurrent sets up numerous side frequencies. Where frequency shift signalsare used considerable band width reduction is obtained. This is ofconsiderable advantage and an object of my improved system is furtherreduction of the band width occupied by the frequency shifted currents.This is accomplished by supplying a low pass pi-section filter betweenthe keyed tone converter and the reactance tube input. This filtershapes the signal wave and controls the rate of build-up of the keyingcharacteristics and reduces the sideband spread and thereby utilizesless band width for the same keying speed and thus provides a reductionof adjacent sideband interference.

The extent of swing of the reactance tube modulated oscillator dependsupon the variation of current through the reactance tube and this inturn depends upon the potential on the reactance tube grid. In myimproved system the keying unit is to supply transmitters operating atthe fundamental frequency or at various multiples thereof. The frequencyshift at the modulated oscillator is then related integrally to thedesired total frequency shift at the output. If the total swing is to bethe same or substantially the same when the multiplication factor ischanged the swing at the modulated oscillator must be reduced. An objectof my invention is to provide simple and improved means for changing thetotal swing at the modulated oscillator an exact and known amount whenthe multiplication factor is changed. This means comprises anattenuation network which may be included in the circuit between thekeyed tone converter and the reactance tube input to reduce thefrequency shift swing.

When it is desired to change the extent of frequency swing or shift insystems of this nature the voltage applied to the reactance tube isvaried more or less negative relative to zero. This not only changes theswing but may also change the center frequency. A further object of myinvention is to provide improved means for recalibrating the modulatedoscillator. In my system the arrangement is such that in the spacecondition the reactance tube is biased to be fully conductive so thatthe modulated oscillator supplies output of lowest frequency. In themark condition the reactance tube is least conductive and suppliesoscillations of a higher frequency representing mark. In order tofacilitate centering of the oscillator frequency I provide apotentiometer arrangement such that half of the steady mark voltage maybe applied to the reactance tube while its output frequency is beingcompared with the monitoring crystal oscillator frequency and themodulated oscillator is tuned to provide zero beat, as indicated bymeter or other means, with the monitoring oscillator.

In an embodiment the center frequency is automatically corrected whenthe control potential applied to the reactance tube is changed. This isaccomplished by simultaneously changing the reactance tube bias.

In an additional embodiment the center frequency is properly reset whenthe frequency swing is changed by changing the reactance in thecontrolled generator circuit.

In describing my invention in detail reference will be made to theattached drawings wherein Fig. 1 illustrates schematically and by blockdiagram, circuit element and circuit connections,

a frequency shift keying unit arranged in ac I circuit. The reactancetube modulates an oscillator 60 which supplies excitation to anamplifier 76 of the cathode follower type, which in turn excitesdifferentially a balanced modulator Hit. The tubes 82 and 82' of thebalanced modulator Hill are also excited cophasally by carrier currentor voltage from a selected one of the two crystal controlled oscillatorunits I20 and Mil. A side band resulting from the balanced modulationprocess is selected by tuned circuit H34 and fed to an amplifier stageI60 and from the amplifier stage to amplifiers and/or frequencymultipliers H0 as desired. The crystal oscillator for monitoringpurposes and for use in telephony and on-ofi telegraphy has beendesignated 2%. The alternating current source for the heaters of thevarious tubes and for the rectifier supplying the direct currentpotentials and for the temperature regulating units have been omittedfor the sake of simplicity. The direct current potential source, notshown, has its output coupled to voltage regulators 221! so that directcurrent potentials of constant amplitude are provided for certain of thetube electrodes in the modulator stage and modulated oscillator circuitsand in the crystal controlled monitoring oscillator 29%. A switching andmetering arrangement for use in making the necessary adjustments andtuning operations is designated generally at 256.

The frequency shift modulation is carried out as follows. Keyedpotentials filteredand modified and used as will be describedhereinafter are applied to the control electrode 30 of a reactance tubeit to vary the conductivity thereof. The reactance tube has its anode 34and cathode 36 shunting a portion of tuned tank circuit 39 of theoscillation generator including one electron discharge system of tube60. The oscillator per se is substantially conventional and will not bedescribed in detail herein. It includes, however, inductance 4|, aportion of which connects the grid and the anode of tube 60 in aregenerative circuit. This inductance is shunted by a plurality oftuning condensers 44, which are of temperature compensating type, andthe entire inductance Al is shunted also by a trimmer condenser C6 inseries with the condenser C40, also used to tune the oscillator. Thegrid bias supply for the triode 53 includes a radio frequency chokeinductance L2 and resistor R3. In the embodiment being describedoscillator 60 operates at about 200 kc. The reactance tube has itsoutput electrodes shunting a portion of the inductance 4| so that thereactance of the tube 40 is included in the tank circuit 39 and takespart in determining the frequency of operation of the oscillator. Thereactance tube 49 is of a type well known in the prior art, having itsanode 34 operating at a first radio frequency voltage by virtue of itsconnection to the tank circuit 39 and its control grid 30 operating atan R. F. voltage in quadrature phase relation to said anode voltage byvirtue of the operation of the phase shifting network includingcondenser C, resistance RI and inductance 21.

The amplifier tube system has its control grid H coupled by condenser C8to the anode of tube 60 to be excited by oscillatory energy developed inthe generator including tube 60. The tube 10 is in a cathode followercircuit with a radio frequency choking inductance L3 and resistor R5 inits cathode return circuit. The cathode follower stage is used to keepthe wave form of the oscillator-amplifier stage reasonably clear ofharmonics. Since the cathode follower has a large amount ofdegeneration, the cathode feedback will tend to maintain thegrid-cathode voltage constant. This effect makes the amplifier morelinear. Moreover, the low impedance output of the cathode output keepsthe R. F. voltage lower due to the low gain and helps prevent feedbackeffect on the oscillator.

The amplified oscillations are supplied from the radio frequency chokinginductance L3 by coupling condenser C5 to the primary winding 13 of atransformer Tl having its secondary winding coupled by a tuned circuit15 in pushpull relation to the first grid electrodes 80 and so' of thetwo tubes 82 and 82' in the balanced modulator I00. The balancedmodulator I00 also has the tube grids B0 and 80' excited in like phaseby crystal controlled oscillations from tuned circuit 83, the inductanceof which forms the secondary winding of tuned trans-former T2. Theprimary winding of the transformer T2 is connected to the anode of thetube I IQ of the high frequency crystal generator I26 and to the anodeof the tube I39 of the high frequency crystal controlled oscillator 2I49. One or the other of these oscillators, here oscillator I20, isoperative to supply oscillations of the fixed desired carrier wavefrequency depending upon the position of selector switch S4. The energyis fed from the high frequency crys tal oscillator through thetransformer T2. The secondary of T2 is tuned to the crystal frequencyand the energy is fed through condensers Cl 0, C! I and CIZ and (M3 tothe grids of tubes 82 and 82' in parallel. The resistors R1 and R8 arehigh impedance grid leaks and prevent the tube from drawing excessivegrid current. The condensers CH CH and Cl! and CH3 tune the secondarywinding of transformer TI to the low frequency (200 kc.) but offer a lowimpedance path for the oscillatory energy fed from the selected crystaloscillator through transformer T2 and. the harmonies of theseoscillations.

The oscillators I and I4!) are not being claimed herein and will not bedescribed in detail. They are, however, very good stable oscillators intemperature controlled units so that the carrier oscillations suppliedto the balanced modulator are of good strength and of substantiallyconstant frequency.

The anodes of the two modulator tubes of the balanced modulator I00 arecoupled differentially to a tuned tank circuit I04 the inductance ofwhich forms the primary winding of a transformer T3. This circuit W4 istuned to pass a side band resulting from intermodulation of theoscillations from the oscillator tube 60 and from the selected crystaloscillator I23. The high frequency oscillations, derived from thecrystal, as known. are phase opposed in the tank circuit I04 and balanceout. The low frequency oscillations supplied from the oscillatorincluding tube 6d are tuned out and the tuned circuit I04 is adjusted tosupply the upper or lower side band. In the embodiment illustrated, theupper side band is used. The selected side band is supplied t thecontrol grid H29 of an amplifier tube I30 and after amplificationtherein is impressed from the output tuned transformer T4 on to a shortline (jack J3 and pin contact P3) and thereby supplied to the amplifiersand frequency multipliers in the transmitter equipment H0. The frequencymultipliers are assumed herein to have a multiplication factor of 4 orof8.

The monitoring oscillator 200 comprises an electron discharge device205having its first two grids coupled in an oscillation generatorcircuit including a crystal X3. The tube includes an additionalshielding electrode 201 which is grounded to shield'the anode 209 fromthe 0scillation generating circuits except through the coupling theretoby the electron stream within the tube. The anode of this oscillatortube is coupled to the primary winding I3 of the transformer Tl undercertain conditions and for certain purposes as will be describedhereinafter. The anode of oscillator tube 205 is also coupled by theprimary winding 13 of transformer TI to a positive source of directcurrent potential through the switch S3 in one of its positions when thecrystal controlled monitoring oscillator 200 is put into use. This willbe discussed in detail hereinafter in the statement of operation of thesystem.

At 250 I have shown a switching arrangement (which has been simplifiedand includes only aportion of the switching arrangement in theembodiment tested) used in regulating, adjusting and tuning the system.The milliammeter Ml may be connected in series with the cathode returncircuit of the reactance tube 40, or with the cathode return circuit ofthe oscillation generator 60, or with the cathode return circuit of theamplifier tube 10; or in series with the oathode return circuit of thetubes of modulator I00, or in series with the direct current screen gridcircuits of tubes 82 and 82', or with the grid circult of the amplifiertube I30, or with the anode circuit of the tube I30, or with theresistances RH! and R20 terminating the low pass filter 2 3 dependingupon the positions of the switches 249' and 253.

While it is believed that my invention has been made clear to thoseversed in the art, the purpose and operation of essential and novelfeatures thereof will now be described in detail. The keying voltagefrom the converter H5 is fed to the voltage regulator tube l 8. Asstated above, this voltage varies from zero in a negative direction, sayto about 180 volts, but may vary more or less due to variations in thepotential used on tubes in the tone generator andkeyer l0. The voltageregulator tube 18 is so poled that when the negative potential at itselectrode ll exceeds a selected amount a limiting action takes place. Inthe embodiment described this tube'is used to limit the negative swingof the keyed pulse to about l volts. The converter it! does not have anylimiter and if the power supply voltage therefor varies the keyer outputvaries and lets the power supply modulate the keying. The tube Hleliminates this effect. The output of the tube It! is connected to aresistor IS in series with an adjustable part of the potentiometerresistor 29 and ground. The input voltage to the reactance tube 40 isadjusted by means of this potentiom eter 2c and thus the total frequencyshift may be set from any value between zero and some upper maximumvalue say, for example, 1200 cycles by means of this adjustment. Thefrequency swing is measured and the potentiometer 20 is callbrated togive the proper frequency shift for the correct potentiometeradjustment. This shift should not depart from the calibration unless thereactance tube 40 should change radically or depart from its normalcharacteristics. By using a monitor receiver and checking the beat noteon mark and space keying, the frequency shift may be checked. In theembodiment being described the frequency shift is set at a total of 850cycles. The output of the potentiometer 20 is fed to switch S2 which isused to include or exclude the resistive pad 28 in or from the inputconnections to the reactance tube 40. Whether or not this resistive pad28 is included depends upon the amount of frequency shift to which theoutput is subjected in the multipliers I'Hl. In the embodimentdescribed, with a total frequency shift of 850 cycles output and amultiplication of 4, the reactance tube and oscillator is adjusted toproduce a frequency shift of about 212.5 cycles total. If a frequencymultiplication of 8 is used at HG then obviously the frequency shiftaccomplished at the modulated oscillator 60 must be divided by 2, or beabout 106 cycles total. With the resistor attenuating pad 28 out theadjustment is made such that the reactance tube 40 modulates theoscillator 60 through a range of 212.5 cycles total. Then themultipliers at I10 have a factor of 4. If the multipliers at l 19 have amultiplication factor of 8 then the switch S2 is moved to its otherposition to include the network 28 in the input circuit of the reactancetube 48. The network 28 consists of a 6 db. pad and the attenuation issuch as to reduce the maximum negative potential on the controlelectrode of the reactance tube 40 to reduce the total here to onehalfits previous negative value so that the output total swing will remainessentially fixed at 850 cycles. Thus, once the potentiometer 20 hasbeen set the same does not need to be reset for different multiplicationfactors unless the total output shift is changed from the assumed 850cycles.

The output of the potentiometer 2i! feeds directly or by pad 28 into alow pass filter 24 composed of condenser 23 and inductance 25, and asection of the condenser C3 (at the lower end of 2?). These connectionsinclude the switch S3 in the position shown so that the voltage 'passedby the low pass filter is supplied to the inductance 2'! and thence tothe control grid 3!! of the reactance tube 40. The filter 24, as statedhereinbefore, shapes the signal wave and controls the rate of buildup ofthe keying characters, thus narrowing the spread of the radio frequencyside bands. The filter is terminated by resistors RH! and R211.

The switch S3 has three positions, one designated Frequenc shift,another designated Frequency calibrate, and the third Phone. In theFrequency shift position it will be noted that the regulated directcurrent potential'is supplied from the positive terminal of the sourceand through one of the voltage regulator tubes 22!] by lead 3! to thescreen grid 32, and by lead 33 to the anode 34 of the reactance tube 40,and simultaneously this regulated direct current voltage is supplied tothe anode of the oscillator tube 60 and the anode of the amplifier tubel0. Then the frequency of the oscillations generated in oscillationgenerator tube an is swung by the keying voltage including the directcurrent component through a first range if switch S2 is in the positionshown, or through a range half as J the resistor Bit and Wide as saidfirst range if the switch S2 is put in the other position. The plate andscreen voltages are regulated by means of voltage regulator tubes Vt andV9. The 200 kc. oscillator including tube 8%? is stabilized for ambienttemperature variations and also for voltage varialions. The other halfof this tube, 1. e., section ill, is used as an amplifier. The amplifier10 is loosely coupled to .the oscillator 68 through condenser C8 and theoutput is taken from the cathode impedance L3 through coupling condenserC5.

In the Calibrate position of switch S3, direct current voltages aresupplied as described above and also by leads 35 to the anode and screengrid of the crystal controlled monitoring oscillator 290. In theCalibrate position then the direct current modulated oscillator 86, theampliher it, and the monitor crystal controlled oscillator 268 are alloperating. However, in this position of the switch the control grid ofthe reactance tube ill is connected by resistor RI and inductor 2i to apoint between resistors RI9 and RZ J so that only half of the keyingvoltage on steady mark (1. e., one-half of the highest negative voltage)is supplied to the reactance tube. If the tube 4E3 characteristic isabout linear, now the frequency at which the oscillator til operatesshould be the desired center frequency, in the example given, 200 kc. Tocheck this frequenc the 290 kc. frequency shift oscillator is adjustedor tuned by means of trimmer condensers C5 and CM! if necessary toestablish zero beat with the crystal oscillator 200. For this adjustmentthe transmitter need not be on the air. The intensity of the currentflow shown by the milliammeter MI, included by lead 89 and switches 259and 253 in the cathode return circuits of the balance modulator tubes 82and 82, indicates zero beat. This centers the carrier frequency of thefrequency shift half way (if the reactance tube is linear) between markand space frequency.

When the switches are in the position shown R29 at the terminal of lowpass filter 2d are connected to ground through lead 2? and meter M l.Other switching means (for the sake of simplicity not shown) opens theground connection to R59 and R283 at the point 29. The meter Ml may beconnected in series with resistors Bit and R26 and ground to indicatethe keying current flowing in R19 and R28, thus giving the keyin voltageapplied to the grid 36 of the resistance tube to. In fact this may beused as an indication of frequency shift for setting the potentiometer28, so that the frequency shift could be calibrated against keyingcurrent on mark condition.

The reactance simulated by tube s8 depends on the current therethrough.The current therethrough depends on the grid potential. In order tofacilitate adjustment of the circuits to cause the tube to operate atthe middle of the linear portion of its characteristic curve, its

cathode may be connected to ground through lead 53 and switches 2&9 and259 instead of directly as shown at 35. In the embodiment usedadditional switching means, not shown, opens the ground connection at atwhen the connections through lead 43 and switches 249' and 253 arecompleted to put the meter MI in the cathode return circuit of thereactance tube 40. Now the adjustments at 2Q can be made to get thedesired maximum currents through the tube (observations made at M!) toget the desired reance cha e and fre uen y swine. The additi nalswitching means ha be n omi t n rder to simplify the drawin s. he chodes 53 and 53 of tubes I50 and III are also arranged to be onnected.throu h meter M by l a 55 and 65 respectively, and switches 249 and 253.When s on ected. the direct ound nne tions for these cathodes are brokenat 51 and 51 by the swit h ng means (not shown). escri d a ove, which iscontrolled simultaneou ly with switches 249 and 253. In the shown po-. stion of hese switche the ground connectio for cathodes 53 and 53 arecompleted at 51 and 61.

Similar means switches 249 and including lead 89 between 253 and-thecathodes of the balanced modulator tubes 82 and 82 and the switch (notshown) contact 84, and leads I03 and 105 between switches 249 and 253and the screen grids of balanced modulator tubes 82 and 82 and theswitch (not shown) contacts I01 and I99, and lead lI'I between the gridI29 and switches 249 and 253, and switch (not shown) contact H and leadI41 between the anode and tuned circuit I53 and switches 249 and 253,and switch (not shown) contact I49 and lead 2IlI between'the cathode ofthe monitor oscillator 200 and switches 249- and 253 and switch (notshown) contact 293 and lead 21' between the resistance R20 and themeters 249 and 253 and switch (not shown) contact29, are provided fortuning and adjustment of the balanoed modulator I00, the amplifier I90,the monitor oscillator 200, and for adjustment of the potentiometer 29"and. 28.

On Phone position of switch S3 only the crystal controlledoscillator 290is excited and the transe mitter frequency control is supplied by thiscrystal oscillator and the crystal oscillator at I20 or I40. Thisposition may be used if amplitude phone or amplitude keying is desired.The trans.- mitters may be equipped with amplitude tele: phony or insome cases amplitude (oneoff) keying may be desired. The amplitudekeying is usually accomplished in a later stage by methods well known tothe art. The amplitude telephony may be accomplished in the last (outputstage) or one of the other stages of the transmitter.

When the extent of frequency swing is changed by say adjustment ofpotentiometer 29 or by including network 23 in the reactance tube inputthe center frequency of operation of the system is changed. In thisrespect it may be kept in mind that the minimum negative voltage (zeropotential) applied to the input is fixed. To further improve systems ofthis type means for automatically centering the carrier frequency isprovided.

Automatic centering of the carrier frequencymay be accomplished ifabias'voltage is applied to the reactance tubev through a potentiometerwhich is mechanically coupled to frequency shift adjustment 29. When thefrequency shift cone trol is varied or when the selector switch (XL-X8)is changed age is applied to the grid of the reactance tube to keep thecarrier essentially centered. In Fig. 2 I illustrate a method ofapplying a bias to the reactance tube control grid in the properrelation, to keep the carrier frequency essentially centered. In thisembodiment reference nu merals corresponding to those used in Fig. 1 areused in so far as possible. Resistors RIM and RIOI of which RI D0 is apotentiometer ganged with potentiometer 20 have been added with BI 00connected by switch S I 02 and part of source the proper correctingvolt-i 3.90 to ground. or by way of source 300 to ground. The tap onEH10 i conne ted by swi ch W3 to a po nt between res stances R162 andR103 or to a tap on EH13. B492, and BN3 are in series with RIB. Thecontacts of S3 in the direct current supply circuits or the Various tubeelectrodes etc, other than that used between the reactance tubeand ign linpu nd the meter M nect ons by switches 249 and. 2 .3 h ve been omittedfor the s ke of simplicity- When the switches are in the positions shownthe grid circuit of tube 48 includes RI, 2?, RIB, EH12 and all of RIMand also part of RIOS shunted b an adjustable portion of RIIlI'l inseries with BiIiI andsource 309. The resistor RIIJI and variableresistor RIUO are in series of a value sufliciently high, so that thecurrent through R I 93 is independent of the value of BI 93. Thepotentiometer 20 is mechanically connected to R IIIQ so that the bias isvaried correctly as the frequency shift adjustment 29 is changed. As thefrequency shift is made smaller, bias is applied to the reactance tubeto recenter the carrier. Switch SHJZ is ganged with switch S2, so thatwhen the input signal is twice (the pad .51 out). twi the normal. bias iappl ed to th grid of the reactance tube. It is not necessary to haveSW3 as illustrated and this switch may be deleted from the drawing. Thenthe connection from R499 goes directly to resistor Rlfl3. However, ifdesired, switch SI93 may be included (as shown) and be ganged withswitch S2. Then w t h 92 may he. removed and Source 309 included betweenRI!!! and ground.

Another method is to have a small trimmer ndenser in para lel with comechanically coupled to frequency shift adjustment 20, so ar-. rangedthat when the frequency shift adjustment is changed the carrier. isshifted to provide a fixed center frequency. A small fixed cone densermay also be switched in and out of the circuit when switch S2 (X I-X8)is changed. This embodiment is illustrated in Fig. 3. When thepotentiometer is adjusted to change the frequency shift the condenserCIIIG in shunt to condenser C6 is adjusted to center the carrierfrequency. When the switch S2 is thrown to the lower position to put thepad 28 in the input circuit the condenser Cid! heretofore in shunt tocondenser C5 is removed from the oscillator circuit to increase theoscillator frequency.

Another method is to use pushpull D. C. input signals, say plus for markand minus for space. Pushpull reactance tubes may then be used and tocenter the carrier it is only necessary to remove the input signal andthe reactance tubes re en. at their normal carrier and the frequencyshift for equal mark and space voltages. Thepushpull reactance tubesalso make the frequency shift system less susceptible topower supplyvariations in the unit.

In operating the system the procedure described above and the followingprocedure may be followed.

, After the set is turned on the operator will of course know and selectwhich of th high frequency crystal controlled oscillators I29 or I49 heis to use. This selection is made by use of switch S4. Then the switchS3 is placed on the frequency shift position and the crystal tuning dialon condenser CI'5 in circuit 83 is set at the proper position orapproximately there. This circuit is tuned for maximum modulation asdenoted by maximum cathode or screen grid ourrent onthe meter MI,switches 249 and 253 being reactance tube.

1 l 7 turned to the proper position. The modulator frequency of theoscillatory energy with respect tuning dial on the condenser CH} ofcircuit I04 to a mean or average frequency, means for adis then placedat the proper setting. Curves supjusting the value of said applied pulseenergy plied with the unit may be used to approximately to therebychange the extent of said frequency determine this setting and the dialis then 5 shift, means for recentering the mean or average slightlytuned to obtain maximum output which frequency of operation of saidoscillation genmay be denoted by the grid current in the tube eratorwhen the value of the applied pulse en I30, its grid return circuit thenbeing connected ergy is changed comprising an adjustable reby switches24:) and 253 through the meter Ml. sistance in the bias circuit for saidcontrol elec- This circuit, which in the embodiment described trode, andcommon control means for said last is tuned to the upper side band, mustbe caretwo means.

fully tuned and is quite sharp. The output tun- 5. In a signallingsystem, in combination, an ing condenser C22 is then properly set and inoscillation generator circuit wherein oscillatory doing so the anodecurrent of tube 30 as indienergy appears, a reactance tube havingelecca-ted on the meter MI may be observed in maktrodes in shunt to atleast a part of the gening this adjustment. The input keying signalerator circuit to tune the same, said reactance is then placed on markcondition to apply a tube having a biased control grid, the magnisteadypotential equivalent to mark potential tude of the energy on whichcontrols its reactive across the resistor 20. The frequency shiftadeffect, connections .to said control electrode for justment bypotentiometer is then made to applying pulse energyto said electrode themaggive the desired frequency shift as indicated by nitude of which iskeyed between first and seccurrent through the meter Ml when the sameond values in accordance with signals to vary is connected in serieswith th resistors RIB and the frequency of the generated oscillatoryenergy R213. The switch S2 is then set in the X4 or X8 with respect to aselected value, an adjustable position, X4 indicating that amultiplication resistance for changing the magnitude of the pulse factorof 4 is used, and X8 indicating that a energy to thereby change theextent of variamultiplication factor of 8 is used. The control tion ofthe oscillatory energy with respect to said switch S3 is now placed inthe Calibrate position selected value, means for recentering the freandby adjustment of the condenser CB and/or quency of operation of saidoscillation generator C 36 the oscillators 60 and 260 are brought intowhen the magnitude of the pulse energy is changed zero beat, thiscondition being indicated by slow comprising an adjustable resistance inthe bias variation of current in the meter MI when the circuit of saidcontrol grid, and a common consame is in series in the return circuit ofthe trol for said adjustable resistances. modulator tubes 82 and 82'.Now the switch S3 6. In a signalling system, in combination, anv is puton the frequency shift position and the oscillation generator circuitwherein oscillatory steady mark signal potential is removed andkeyenergy appears, a reactance tube having elecin signal put on. trodesin shunt to at least a part of the generator What is claimed is: circuitto tune the same and control the generator 1. In a signal-ling system,in combination, an frequency, said reactance tube having a biasedalternating current circuit means for causing 0 control electrode thepotential on which controls oscillatory y 0 flow 111 clrcuit, a itsreactive efiect, connections to said control ietit pitt titii trftiitiit thi tifie aii elem applying Pulse energy Said c e S reactance tubehaving a biased control grid, the 33 elgctrode gg g g i fi Valuemagnitude of the energy on which controls its Oug K V ue a 1rd Va be maccord reactive effect, connections to said control grid 9 Wlth slgnalsshlfi the frequency of the for applying pulse energy to Said controlgrid the oscillatory energy with respect to a mean or avermagnitude ofwhich is keyed between first and age frequency, means r dj s n t e Va ueO second values in accordance with signals to vary said applied pulseenergy to thereby change the the tuning of the circuit between twolimits extent of said frequenc shift, means for recensymmetrically relto a n er fr q n y, an tering the mean frequency of operation of saidadjustable resistance in said connections for oscillation generator whenthe Value of the apchanging the extent of the separation between saidfirst and second values of the pulse energy plied pulse energy ischanged compnsmg an to thereby change the extent of variation of thejustable reactance in said oscillation generator Said tuning of Saidcircuit, and means for circuit, and common control means for said lastmultaneously recentering the tuning of said cirtwo meanscuit at saidcenter frequency when the extent of In a Signalling System, inCombination, o separation between said two values of the pulse cuitwherein oscillatory energy flows, a reactance energy is changed. tubehaving input electrodes and having output 2. A system as recited inclaim 1 wherein said 1 t d coupled t 5 last means is a second reactanceelement in said th tuning th f nd thereby trolli the circuit. timing ofthe said oscillatory energy, a source of A System as recited in Claim 1wherein Said direct current potential which varies between two lastmeans is a controllable bias source for said values connected to theinput electrodes of said tube to vary the current therethrough andtherei. In a signalling system, in combination, an 5 by vary the timingof the oscillatory energy beoscillation generator circuit whereinoscillatory tween two values, frequency increasing means energy appears,a reactance tube having eleccoupled to said circuit for increasin therange trodes in shunt to at least a part of the generator between saidlast named two values, direct ourcircuit to tune the same, saidreactance tube rent and alternating current potential attenuathaving abiased control electrode the potential ing means, and switching meansarranged to inon which controls its reactive effect, connections cludesaid last named means in the connection to said control electrode forapplying pulse enbetween said source of direct current potential ergy tosaid control electrode which varies from and the input electrodes ofsaid reactance tube a. first value through a mean value to a third whenthe multiplication factor of the frequency value in accordance withsignals to shift the increasing meansismade larger.v

aid circuit for controlling 8. In a signalling system, in combination, acircuit wherein oscillatory energy flows, a reactance tube having inputelectrodes and having output electrodes coupled to said circuit forcontrolling the tuning thereof and thereby controlling the timing of thesaid oscillatory energy, a source of direct current potential whichvaries between two values, one of which is fixed and the other of whichmay vary in an undesired manner, coupled to electrodes of said tube tovary the current therethrough and thereb vary the timing of theoscillatory energy between two values, a voltage regulator tube in shuntto said source and poled so as to limit variations of said otherpotential so that the timing of said oscillatory energy is variedthrough a fixed range, frequency converting and frequency multiplyingmeans coupled to said circuit for multiplying the range through whichsaid oscillatory energy is varied, a direct current potentialattenuator, and a switch to include said attenuator in the connectionbetween said source of direct current potential and the input electrodesof said reactance tube when the multiplication factor of the frequencyincreasing means is made larger.

9. In a signalling system, in combination, a circuit wherein oscillatoryenergy flows, a reactance tube having input electrodes and havin outputelectrodes coupled to said circuit for controlling the tuning thereofand thereby controlling the timing of the said oscillatory energy, asource of direct current potential which varies between two values, oneof which is fixed, coupled to the input electrodes of said tube to varythe current therethrough and thereby var the timing of the oscillatoryenergy between two values, a voltage limiter coupled across said sourceand poled to fix the other value between which said potential varies, tothereby fix said two values between which the oscillatory energy istiming modulated, a low pass filter in the coupling between said sourceand the input electrodes of the reactance tube to reduce sidefrequencies produced by variations of said potential, frequencyincreasing means coupled to said circuit for increasing the frequencyrange through which the oscillatory energy is modulated, a controlpotential attenuator, and means to introduce said attenuator in theconnection between said source of direct current potential and the inputelectrodes of said reactance tube when the multiplication factor of thefrequency increasing means is made larger.

10. In a signalling system, an oscillation generating circuit, areactance tube having output electrodes associated therewith forcontrolling the frequency thereof, a source of modulating potentials ofvarying value coupled to the input electrodes of said reactance tube tomodulate the frequency of said circuit, means for multiplying thefrequency of the output of the said modulated circuit, a potentialattenuator, and a switch for introducing said potential attenuator inthe connection between said source of modulating potentials and theinput of said reactance tube when the multiplication factor is changed.

11. In a signalling system, in combination, an oscillation generatorcircuit wherein oscillatory energy appears, a reactance tube havingelectrodes in shunt to at least a part of said circuit to tune the sameand control the generator frequency, said reactance tube having acontrol grid the potential on which controls its reactive effect,connections to said control grid for applying pulse energy to saidcontrol grid the magnitude of which varies from a first to a secondvalue in accordance with signals to vary the effective reactance and thefrequency of the oscillatory energy, and means for centering thefrequency of operation of said oscillation generator comprising acrystal controlled oscillator operating at the desired center frequency,means for applying a potential equal to one-half said second value tosaid control grid of said reactance tube, and means for then adjustingthe frequency of the modulated oscillation generator until it equals thefrequency of the crystal controlled oscillator.

HALLAN E. GOLDSTINE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,054,883 Schlesinger Sept. 22,1936 2,176,168 Bellescize Oct. 17, 1939 2,279,659 Crosby Apr. 14, 19422,285,044 Morris June 2, 1942 2,380,959 Frankel Aug. '7, 1945 2,339,608Alvira Jan. 18, 1944 2,351,193 Crosby June 13, 1944 2,379,614 TunickJuly 3, 1945 2,398,793 Magnuski Apr. 23, 1946 2,401,629 Finch June 4,1946 2,403,358 Gerhard et a1. July 2, 1946

